In the past it has been suggested that solid polymer electrolytes can be successfully produced for use in various forms of electrochemical devices. Certain prior art examples for producing these solid state electrolytes are contained in the patents to Lee et al 4,830,939, May 16, 1989 and 5,238,758, Aug. 24, 1993; and to Olsen 5,384,313Jan. 24, 1995. In these patented disclosures prepolymers or monomers and prepolymer mixtures with an electrolyte salt have been spread on various surfaces to be polymerized to produce the desired solid polymer electrolyte.
The two Lee et al references teach the art how to produce either a free standing solid polymer electrolyte film or an electrode half element by first producing a radiation curable mixture of a monomeric or prepolymeric radiation polymerizable compound, a radiation inert ionically conducting liquid, and an ionizable alkali metal salt that can be poured into a mold or coated onto a surface previously coated with a release agent. The mixture is then cured. The resulting product is the desired free standing electrolyte film having a film thickness that can vary from about 15 to 100 microns thick. Or in another form the film can be polymerized in situ on a half element by being cured on a metal foil that has been coated with this radiation curable mixture. The coating of the resulting viscous mixture on the metal foil is placed on the foil by any known method such as a rod coating, roll coating, blade coating, extrusion step or other such procedure to lay down a coating that can then be cured on that surface.
Olsen described a similar procedure for making a device with a solid polymer electrolyte for use in an electrochemical device. He teaches the process of producing an electrolyte precursor comprising of a solid matrix-forming monomer and/or partial polymer thereof in a solvent capable of also dissolving an electrolyte salt. This solution is coated onto an electrode to form a layer of from 25 to 120 microns thick. This layer is then covered with a protective sheet to prevent the solvent used in the precursor mix from evaporating, then curing the electrolyte precursor by radiating the mixture on the electrode and then removing the cover. It is stated that the precursor is applied to the electrode by any conventional technique such as by extrusion, blade coating, silk screening or other well known techniques to produce a polymerizable layer having a thickness ranging from between no less than about 25 to as thick as 120 microns. The minimum thickness is specified to be of that thickness in order to ensure that the coating is pinhole free and the ultimate 120 micron thickness is specified because it is said that a greater thickness would increase the thickness of the final electrochemical device and thus decrease the energy density of that device.
In all of these known teachings the starting polymeric mixtures produce a viscous liquid electrolyte precursor adapted to be extruded or blade coated onto a support prior to being polymerized to form the ultimate solid polymer layer. Various additives are suggested for inclusion in the precursor, such as solvents and electrolyte salts, but the basic precursor composition being prepared for laying down the ultimate layer that is to be polymerized to form the ultimate solid polymer electrolyte, is a somewhat viscous solution. Since one or more components of the starting materials mixtures is a polymer or prepolymer which has relatively high molecular weight, the resulting electrolyte precursor is too viscous to spread using an economical and practical method such as spraying. However, due to the high viscosity, equipment such as extruders or blade applicators is necessitated to spread the viscous precursor.